In preparing optical integrated circuits, for realizing miniaturization and higher integration, it is effective to use a material having a high refractive index for a core of an optical waveguide. Particularly, as Ta2O5 is a material having a high refractive index and transparency, it is possible to form an optical waveguide of a low loss (non-patent document 1). Further, it was further studied to find a shape for preventing substantial loss by adjusting the structure of the waveguide (non-patent document 2). If a waveguide made of Ta2O5 and of a low loss is realized, it becomes possible to make a size of an optical integrated circuit considerably smaller compared with that realized by using a silica-based optical waveguide. Further, if a circuit of a large integration is realized, it can be established to provide an optical integrated circuit of improved function.